Well my idea was that this car was gas powered and has been retrofitted into Nuke powered so there's another use for the gas tank(or at least the space it occupied). Now there's a pipe/cables leading to where a gas cap would have been.
I searched for the right rust/dirt textures that would work on this car and put them on a seperate Multiply Layer, then modified it from there. The thing is, you have to put the rust/dirt where it makes sense, and is believable. In this case I put rust around the gas cap and at hinges or seams/edges and dirt throughout. Hope that helps.